Frequency tunable band-stop filters are widely used to remove undesirable signals. Most band-stop filters are reflection type filters, meaning that at the frequency where the filter rejects a signal, that rejected signal is reflected back to the input. This is undesirable for many reasons. For example, consider a microwave or radio frequency receiver that receives a signal through a RF port.
If the receiver utilizes a frequency mixer, it most likely uses a band-stop filter to rejected unwanted signals. When the rejected signal is reflected by the band-stop filter, it may leak into the RF port. Once the rejected signal leaks back to the RF port, it may be remixed into the input signal, which is known as self-mixing and is very undesirable across most applications.
Recently, reflectionless or absorptive band-stop filters have been proposed which overcome the basic limitation of reflection type filters. Through the design of lossy resonators, reflectionless type band-stop filters can achieve a theoretically infinite amount of attenuation. The current, state-of-the-art reflectionless filters are acceptable for low frequency applications. However, for high frequency applications, realization is difficult due to the parasitic and self-resonance effects from the large number of hardware elements required.
Band-stop filters have been implemented for use at high frequencies using L-resonators and they have become fairly common due to their ease of design and implementation; however, they are reflective in nature. Accordingly, for at least the foregoing reasons, there exists a need for a reflectionless band-stop filter that is easy to design and implement.